R. WAACKAND M. A. DORAN
148
Vol. 67
SOLVENT EFFECTS I N THE ELECTRONIC SPECTRUM OF ORGANOLITHIUM. COMPOUNDS BY R. WAACIC AND M. A. DORAN Eastern Research Laboratory, The Dow Chemical Company, Frarningham, Massachusetts Received J u l y 6 , 1962 We have found that the absorption inaxhum in the electronic spectruin of di- or triphenylmethyllithium compounds is shifted to longer wave lengths by more polar solvents. For the most thoroughly studied species, 1,l-diphenylhexyllithium,the shifts in absorption maximum are approximately proportional to the solvent dielectric constant. Two factors are considered to account for the observed red shift in absorption maximurn: first, a decrease in the energy required for the electronic excitation resulting from increased solvation of the excited state due t o the polarizability and dipole orientation of the solvent; and second, the more polar (basic) solvents promote ionization of the carbon-lithium bond, raising the ground state energy of this electron pair relative to the excited state. A solvent induced shift in the absorption spectrum cannot be considered to be a general phenomenon for all organolithium compounds. The position of the absorption maximum of monophenylmethyllithium species, e.g., polystyryllithium, poly-or-methylstyryllithium, benzyllithium, and or-methylbenzyllithium is insensitive to solvent. A lack of increase in dipole moment on electronic excitation of these species or a preference of these less sterically hindered species toward self solvation is suggested to be responsible for the insensitivity of their spectrum t o solvent. Consideration of solvation effects can improve the agreement between the spectrum of the cation and anion of a given species or the correlation of the absorption with calculated transition energy.
Ultraviolet spectra of polar molecules usually are altered by changes in polarity of the solvent media. In general, the K-bands of conjugated compounds are shifted to the red by more polar solvents.2 A report of the spectrum of the polar organolithium compound, polystyryllithium, showed the spectrum to be insensitive to solvent changes, i.e., in tetrahydrofuran and b e n ~ e n e . ~Our measurements of the spectrum of this species also show it to be solvent insensitive. In addition we have found this same solvent insensitivity in the spectrum of other monophenylmethyl organolithium species such as benayllithium, a-methylbenzyllithium, and poly-or-methylstyryllithium. On the other hand, we have found the electronic spectrum of diand triphenylmethyllithium compounds to be very sensitive to solvent. For these species a more polar solvent produces a pronounced red shift in the absorption maxima. There appears to be a linear relationship between the frequency of the absorption maximum and the dielectric constant of the solvent for 1,l-diphenylhexyllithium in the seven solvents examined. These experiments indicate that a lack of solvent sensitivity is characteristic of the electronic spectrum of monophenylmethyl organolithium species, whereas a pronounced solvent dependence is typical of the spectrum of di- and triphenylmethyllithium species.
Experimental The spectra were measured in a closed cell apparatus4consisting of a 0.2-cni. path length quartz cell, sealed to a side arm of a 25ml. flask. This was connected to a high vacuum system via an attached vacuum stopcock for alternate evacuation and flushing with argon. The spectra were measured under an argon atmosphere. A removable 0.18-cm. quartz spacer was sealed in the side arm, thus permitting a change in cell path length. This apparatus is illustrated elsewhere .6 A Cary Model 14 spectrophotometer was used to record the spectrum. The standard procedure followed t o form 1,l-diphenyl-nhexyllithium, polystyryllithium, and poly-a-methylstyryllith(1) Preliminary communioation: R. Waack and M. A. Doran, Chem. Ind. (London), 1290 (1962). (2) L. Lang, Ed., "Absorption Spectra in the Ultraviolet and Visible Region," Academic Press, New York, N. Y., 1961, p. 53. (3) D. J. Worsfold and 9. Bywater, Can. J . Chem., 38, 1891 (1960). (4) The high reactivity of organolithium compounds requires the use of a n absorption cell t h a t prevents contact with air or moisture. ( 5 ) E . Waack a n d M. A. D o r m . to be published (1963).
ium was t o add n-butyllithium to a solution of the corresponding olefin in the desired solvent. The solvents were distilled from lithium aluminum hydride or liquid sodium under argon. The olefins, 1,l-diphenylethylene, styrene, and a-methylstyrene were vacuum distilled from calcium hydride and stored under argon. With or-methylstyrene, the concentration of the monomer was well below its equilibrium value.6 Hence, the initiation product should predominate. A stoichiometric amount of n-butyllithium to olefin wm used, except with styrene where excess olefin wm used to ensure consumption of the n-butyllithium. If the previous procedure is reversed, there are no differences in the spectrum, but in the standard procedure the spectrum of the olefin was recorded and its subsequent disappearance observed on addition of the n-butyllithium.7 Butyllithium in hexane solution was obtained from Foote Mineral Corporation. A small amount of hexane therefore was present in each experiment. This should not affect the absorption spectrum 8,s the active center is expected to be preferentially solvated by the more abundant higher polarity solvents. Gastight syringes (Hamilton Co.) were used to transfer the solvent, olefin, and organolithium solutions. Benzyllithium was prepared from tribenzyltin chloride and phenyllithium in ether,* chilling the reaction after 2 hr. to precipitate the tetraphenyltin and filtering. a-Methylbenzyllithium was prepared in an analogous manner from a-methylbenzyltriphenyltin.$ The yields of tetraphenyltin in these reactions were 95 to 100%. Carbonation of benzyllithium resulted in a 70% yield of phenylacetic acid. Phenyllithium was prepared from bromobenzene and lithium metal in ether at 10'. Carbonation produced benzoic acid in greater than 90% yield. Triphenylmethyllithium was prepared in tetrahydrofuran from triphenylmethyl chloride and lithium metal at O", filtering after 4 hr. Benzyllithium and a-methylbenzyllithium, which were prepared directly in the cell apparatus by reacting stoichiometric amounts of phenyllithium (or butyllithium) with the appropriate tin compound, gave long wave length absorption identical with the previous preparations. The tetraphenyltin so produced does not interfere although its spectrum is observed. The rate of the transmetalation reaction is directly observable from the build-up of lithium compound. It is very rapid in T H F but becomes progressively slower in the less polar solvents, analogous to the olefin addition reaction.6 Each experiment was completed by the addition of several drops of ethanol, destroying the long wave length absorption of the carbanion and producing the spectrum for the hydrocarbon. (6) H. W. McCorrnick, J. Polymer Sei., 26, 488 (1957). (7) I n THF and dimethoxyethane, maximum intensity is obtained immediately. I n the less polar solvents, the addition reaction is progressively slower and the disappearance of the olefin can be followed. For example, see A. G . Evans and D. B. George, J. Chem. Soe., 46.53 (1961). (8) 1% Gilman and S. B. Rosenberg, J . Org. Chem., 24, 2063 (19.59). (9) This tin compound was kindly supplied by Dr. F. C. Leavitt and Miss Prisoilla A. Carney of this Laboratory.
Jan., 1963
SOLVENT EFFECTS IN ELECTRONIC SPECTRUM OF QRGANOLITHIUM COMPOUNDS Results
149
260
The position of the principal absorption maximumi of 1,1-diphenyl-n-hexyllithium(I) in seven different solvents is listed in Table I. In the five most polar solvents, a weaker absorption band at shorter wave length is also evident, e.g.,, , ,A 315 mp in T H F which shifted to 300 mp in isopropyl ether. This band is not observed in the two low polarity solvents. It is apparent that a more polar solvent medium causes a bathochromic shift in the absorption maximum. The 81 mp shift between the most polar and least polar solvent is unusually large. The ionic strength of the solution apparently is of lesser importance, because excess amounts of butyllithium have no effect on the position of the absorption maximum of I.
=.
250
I
0,
240
230 I
8,
220
w'
210 200
SOLVEWT DIELECTRIC COWSTAWI
Fig. 1.-Absorption
maximum in ultraviolet spectrum of 1,ldiphenyl-n-hexyllithium.
TABLE I (11). This is reported to show maximum absorption THEPO~ITION OF THE ABSORPTION MAXIMUM IN THE ELECTRONICa t 475 and 410 mp in diethyl ether,'O whereas we SPECTRIJM O F 1,1-DIPHENYL-n-HEXYLLITHIUM I N SOLVENTS O F observe the absorption maximum of I1 at 500 and 425 DIFFERENTPOLARITY mp in T H F ; thus, the absorption of I1 also experiences Solvent
Primary ahahsorption max., mp
Dielectric constant ( 2 0 O )
Tetrahydrofuran 496 7.58' b 1,2-Dimethoxyethane 495 Diethyl ether 438 4 335* Isopropyl ether 428 3.88" n-Butyl ether 435 3.06 (25')' 426d 2 286" Benzene %-Hexane 415 (broad) 1.890° a F. E. Critchfield, J . A. Gibson, Jr., and J. L. Hall, J. Am. Chem. Sac., 75, 6044 (1953). The dielectric constant of 1,2dimethoxymethane has been determined as 5.50 and that of T H F as 6.00: J. L. Downs, J. Lewis, B. Moore, and G. Wilkinson, J . Chem. Sac., 1367 (1959). The small difference between the dielectric constant of these two solvents corroborates our observation of their similar solvation properties. However, the above value for the dielectric constant of THF is quite different from that determined by Critchfield, et al." We have chosen touse thislattervalue. A . A. Maryott andE. R. Smith, "Table of Dielectric Constants of Pure Liquids," N.B.S. C m . 514 (1951). dThis measurement in benzene agrees with the recent report for the spectrum of I in benzene, i.e., Xmax 428 mp: A. G. Evans and D. B. George, J. Chem. Soc., 4653 (1961). e P. Debye and H. Sack, "Constantes Dielectriques Moments Electriques," Hermann and Cie, Paris, 1937.
The dependence of the position of the absorption maximum, in reciprocal centimeters, on solvent polarity, as expressed by solvent dielectric constant, is illustrated in Fig. 1. The linearity of Fig. 1 might be expected from the relationship AE = hv, providing the solvent dielectric constant is a consistent measure of the extent of solvent interaction on the transition energy. The faster rate of development of absorption intensity in the higher dielectric media is also illustrative of such solvation a b i l i t ~ . ~ The slope of the line measures the average lowering in electronic transition energy by solvent interaction, per unit dielectric constant. One might expect that extrapolating to zero dielectric constant would best estimate the transition energy in the absence of solvent effects. The limits of =k5 mp shown in Fig. 1 are an estimate of the possible variation in locating the maximum of these rather broad absorption curves. The scatter shown by the data is perhaps caused by steric effects, which are not expressed in the dielectric constant, but which are a prominent factor in the ability of the medium to solvate. There is no simple relationship between the absorption maxima and solvent refractive index. Another species similar to I is triphenylmethyllithium
a red shift with increasing solvent polarity. The spectrum of the diphenylmethyl anion (the gegenion is not specified, but is assumed to be Na) is reported to have an absorptioq maximum at 435 mp in diethyl ether,l0which is almost identical with our observation of I in ether. The small 3 mp difference is explicable in terms of the usual bathochromic effect of substituting an alkyl group for a h y d r ~ g e n . ~ , ~ In liquid ammonia, the position of maximum absorption of diphenylmethyl potassium is reported as 440 mp,ll strikingly similar to that in diethyl ether. I n view of the high dielectric constant of ammonia, however, (22.5 a t -33.4', see Table I, ref. e) it is apparent that the relationship of Fig. 1 does not hold; this is perhaps due to the larger gegenion or the dual solvating behavior of NHa, i.e., it can act as a base to solvate the cation or H-bond with the lone pair electrons of the ionized C-Li bond. The spectrum of the alkali metal-benaophenone ketyl (111) is also reported to experience a red shift (of the order 30-35 mp) on changing solvent from 1,4-dioxane (ez5' 2.2Ogc) to THF.12 The relectron system of IT1 is quite similar to that of I. The electronic transition observed for this radical-ion system is suggested to be the excitation of an electron from the lowest antibonding n-orbital to one of higher energy. l 3 In contrast to the di- and triphenylmethyllithium species, the electronic absorption spectra of monophenylrnethyllithium compounds are comparatively insensitive to solvent. The first reported example was poly~tyryllithium.~We observe this species to have a single well defined absorption maximum a t 335 mp in T H F and similarly at 334 mp in benzene. The only solvent effect is a broadening of the band in the more polar medium. We have measured the spectrum of other monophenylmethyllithium compounds in three different polarity solvents. These data are summarized in Table 11. It is apparent that these monophenylmethyl orgaiiolithium do not show a pronounced solvent sensitivity in their electronic spectra. I n fact there appears to (IO) S. F. Mason, Qua~t.Rev., 16, 336 (1961). (11) I. V. Astaf'ev and A. I. Shatenshtein, Opt. Speetr. (English Translation), 6,410 (1959). (12) H. V. Carter, €3. J. McClelland, and E. Warhurst, Trans. Fas*aday Soc., 66, 455 (1960). (13) B. J. McClelland, ibid., 67, 1458 (1961).
R.WAACKASD M. A. DORAX
150
Yol. 67
TABLE I1 electron from the highest occupied n-orbital to the THEPOSITION OF THE ABSORPTION MAXINIXIN THE ELECTRONIClowest antibonding n-orbital. SPECTRA OB MONOPHENYLMETHYL ORGANOLITHIUM COMPOUNDG The absorption of ultraviolet light by a molecule IN SOLTTEKTS OF DIFFEREXT POLARITY results in its electronic excitation from some ground -Position Solvent
Benzyllithium
THF Et20 Benzene
330 328
of maximum absorption, mpCY-MethylPoly-abenzylhthium methylstyryllithium
335 335 344
340 337 349
be a small opposite (blue) shift in the least polar solvent. Another organo-alkali metal system in which the spectrum does not appear to be solvent sensitive is the alkali metal-aromatic hydrocarbon radical-ions. The major absorption band of sodium-anthracene radicalion is identical in 1,4-dioxane12 and in THF1*”or dimeth0~yethane.l~~
Discussion Although the solvent effects in the electronic spectrum of organolithium compounds (carbon-lithium bond) have not been studied preFiously, the effect of solvents on electronic spectra in general has received considerable attention. Some of the highly conjugated organic dyes show pronounced solvent effects in their spectra.15 The shifts usually are less pronounced than that observed here. For example, phenol blue, which consists of both homopolar and charged zwitterionic resonance structures, undergoes a pronounced red shift in absorption maximum in more polar solvents. On the other hand the spectrum of Bindschellers’ green, which has a formal charge in both the ground state and excited state, is relatively insensitive to solvent changes. These different behaviors are readily explained by the ability of the solvent to stabilize their different resonance structure^.^^^^' The odd alternant organolithium species are a very interesting example of a class of compounds all of similar structure (in having analogous resonance forms) but for which we find individuals that behave in different manners regarding spectral shifts due to solvent. Thus, the explanation here is not simply the preferred stabilization or destabilization by the solvent of a certain resonance structure. I n each of these organolithium species delocalization of the carbon-lithium bond electrons throughout the n-system is a stabilizing factor. This tendency to delocalize the unshared pair, for example in I, is reflected in the high ‘(acidity” observed for the corresponding hydrocarbon. Along with its stabilizing effect, electron delocalization moves the bonding and antibonding n-orbitals closer together.l* Thus, we assume that the observed long wave length absorption in each of these compounds is due to the excitation of an (14) (a) P. Balk, G. J. Hoijtink, and J. W. H. Schreurs, Rec. trar. ehirn., 76, 813 (1957); (b) E. DeBoer and S. I. Weissman, zbzd., 7 6 , 824 (1957). (15) L. G. S. Brooker and R. H. Sprague, J . *4m. Chem. Soc., 63, 3214
(1941).
(16) G. W. Wheland, “Resonance in Organic Chemistry,” John Wiley and Sons, New York, N. Y., 1955, pp. 325-326. (17) 31.J. S. Dewar, Chem. Soc. (London), S p e c . Publ., 4, 75 (1956). (18) I n a n odd alternant molecule as I, the highest occupied orbital is non-bonding, 3 0 these electrons do not contribute t o the resonance energy. It is the lowering of the other z-orbitals (relative t o their position in the nondeIooalized molecule) t h a t produces increased stability. The occurrence of tho non-bonding orbital between the bonding and antibonding orbitals is the reason odd alternants absorb at longer wave length than even alternants, for example see ref. 17, p. 78.
state energy to a more energetic excited state. I n solution, the extent of the solvation of the molecule in each of these two electronic states influences their relative positions in energy. It is apparent that in diand triphenylmethyllithium compounds, the more polar medium reduces the energy difference between the ground and excited states, thus causing a red shift in absorption maximum. We consider that the pronounced red shift in absorption maximum of such species, with more polar solvents, might be explained in two mays which are somewhat interrelated. First, note that the organolithium systems under discussion consist of a polar solute in both polar and non-polar solvent medium. As we mentioned, conjugation bands in electronic spectra frequently are shifted toward the visible by a more polar solvent. This is because the absorbing molecule (to be considered in the classical sense as an oscillating electron) which is imbedded in a dielectric medium polarizes the medium and thus decreases the work required for electronic e ~ c i t a t i o n . ~ ~ ~ ~ ~ The lowering in energy is dependent on the polarizability of the solvent, There may also be a solvent effect due to orientation of the solvent dipoles by the solute. I n the spectrum of a polar solute in a polar solvent, a strong red shift may be expected if the solute experiences an increase in dipole moment on excitation.21 The increase in solute dipole in the excited state results in a gain in solvation energy for this state, relative to the ground state, through increased interaction with the oriented solvent molecules, On the other hand, should the solute dipole decrease during electronic excitation, the oriented solvent would cause a blue shift due to “orientation strain,” which will be superimposed on the polarization red shift.21 Such may be the case for the monophenylmethyl organolithium compounds discussed later. Another factor, the promotion of ionization of the carbon-lithium bond by more polar solvents,22 might also be considered as contributing to the observed solvent effect in the spectra of organolithium compounds. Consider that an organolithium compound may be described, in valence bond language, by a wave function containing contributions from covalent and ionic structures, i.e. $R-Li =
(R-Li)
+
bJ/(ionio)
(ReLi@)
Presumably, in the ground state -ais large whereas in the electronic excited state -bwould be of greater importance. Thus, an increase in ionic character of the ground state, via solvent-promoted ionization, should lower the electronic transition energy (keeping in mind that the transition is suggested to be T+R* of the delocalized electrons). TO say this in another way, bond formation lowers the energy of the carbon-lithium bond electrons, whereas ionization raises the energy of these electrons, relative (19) science (20) (21) (22)
F. A. Matsen, ”Techniques of Organic Chemistry,” Vol. I X , InterPublishers, Ino., New York, N. Y., 1956, p. 695. S. F. Mason, Quart. R e & , 16, 368 (1961). N. S. Bayliss and E. G. McRae, J . Chrm. Phgs., 68, 1002 (1954). D. J. Kelley and A. V. Tobolsky, J . Am. Chem. Soc., 81, 1597
(1959).
to the non-delocalized s t r ~ c t u r e . This ~ ~ concept is analogous to t,he well known blue shift caused by hydrogen bonding solvents on carbonyl n+n*-transiti on^.^* In this latter system, the energy of the nonbonding electron pair is lowered in the ground state hy TI-bond formation, whereas t,herc is little 11-bonding in the cxcitcd The result is an increase in t;ransition energy observed as a blue shift. ’I’he ability of the solverit to promote ionization of the organowhich is lithium compound is related to its ba~irity,~’ dotermined by such properties as permanent dipole arid polarizability as well as steric factors which regulate the closeness of approach or the number of closest solvent molecules. I n Table I, solvent basicity de-’ creases in tho same order as the dielectric constant. l’rom this discussion, the observed red shift in the absorption maximum of di- and triphenylmethyllithium species, with increasing solvent polarity, might be attributed to a decrease in the energy required for t’hc electronic excitation resulting from increased solvation of the excited state due to the polarizability and dipole orientation of the solvent, or the more polar (basic) solvents promote ionizat,ion of the carbon--lithium bond thus raising the ground st-ate energy of this elect’ron pair relative to the excited state. Following the foregoing discussion, two possible explanations may be considered to account for the insensitivity to solvent of the monophenylmcthyl organolithium compounds. The first possibility is that in these solvent insensitive species, electronic cxciLation is not accompanied by an increase in dipole moment, so solvent Orientation would not cause a lowering of the excited state energy. If the dipole momcnt decreases on excitation, the effect of solvent “orientation strain” may he superimposed on the “red!’ polarization shift and if the former dominates, (23) Electron delocalization stabilizes the excit,nd state to a greater vxtcnt than tile ground state, a n d results in a snialler difference in energy betaern these t x o states. Ilond formation of the C-Li bond in analogous in out, of plane, resulting in reto twisting cornponrnts of :k conjugatrd s duced interaction. This usually o a u w s a hypsochroinic shift in a - ~ * transition and a IOSS of intensity. See, f u r t w ~ n i p l ~ A* I, , J. S.1)ewur. “Ytcric lfiTrct,s in C o n j u g a t d Systems.” Aoadcniie Prrsn, Inc., N e w York. N. T., 1Y58. p. 46. (24) 31,Iiaslia, Discirasions Faraday Soc., 9, 15 (1950). ( 2 5 ) 11. AfrConnt.11, J . Chem. I’hgs., 20, 700 (lW2). (26) C. J. t l r i d e y a n d M. Kaslia, J . A m . Chem. Soc., 77, 4LG2 (l!)X). (27) 11. C. Drown and R. .\I. Ailains, ibid., 64, 2.557 (18.12).
a small red shift nith decreasing solvent polarity may be realized. Such a trend is indicated by the data in Table 11. Another approach, which is in keeping with the second explanation of the solvent effect discussed previously, is based on the well known tendency of organolithium compounds to form Thc species which show strong solvent dependence are very bulky, and consequently may be sterically prohibited from cluster formation. The organolithium function of the bulkier specks thus is available for ordination with the smaller solvent molecules. On the other hand, the monophcnyl species should have little steric inhibition to cluster formation, so they preferentially solvate themselves and are less scnsitive to the solvent environment. Thus, we ronsider that the insensitivity of the spectrum of monophenylmethyllithium compounds to solvent may result from a lavk of increase in dipole moment on elevtronir cscitation or a preference of the less sterically hindered spcc+s toward self solvation. One important consequence of these findings bears directly on the correlation of the absorption spectrum of cations, anions, or free-radicals of odd or even altcrnant hydrocarbons with each otherzQ or with transition energies calculated by molecular orbital methods.30 The solvent in which thc speclrum is measured has a pronounced effect on the observed transition energy. It is apparent that in ciomparing the spectrum of the cation or anion of a species, which is solvent sensitive, thc solvent medium of the cation and anion must have similar solvating properties. For example, comparing the spectra of the diphenylmethylcarbonium ion in sulfuric acid (Arnax 429 mp3I) and the anion in isopropyl ether (A,,, 428 mp) gives quite good agreement, but the agreement is not good when the anion spectrum is taken in the higher solvating medium THE’ (A,, 496). The correlation of absorption frequency and transition energy calculated by I-Iuckel M.O. method also may be improved by considering solvent effects. (80-
( 2 8 ) It ( 1 Wittig, F. J. hfeyer, and G. Lanne, Ann., 671, 167 (lYil) (29) AI J. 5. Ileuar a n d €1. C. Longuet-IIiggins, l + o c . Phys. Soc (London), 867, 792 (1954). (30) 4. hticitniesoi, “AMolecularOrbital T h m r y for Organic Cliemiutr,” John Wiley and Sonh Inc , K r w York, K. Y , 1961, p. 228. Jaruzclski and ‘1.Schriesheiin, J O r g Chem , 19, 155 (1954).
